The present invention relates to secondary cells which comprise a can and a rolled-up electrode unit or like cell element accommodated in the can and which are adapted to deliver electric power generated by the cell element from a pair of electrode terminals attached to the can.
In recent years, attention has been directed to lithium secondary cells or batteries having a high energy density for use as power sources for portable electronic devices, electric motor vehicles, etc. Cylindrical lithium secondary cells of relatively large capacity, for example, for use in electric motor vehicles comprise, as shown in FIGS. 11 and 12, a cylindrical can 1 having a cylinder 11 and lids 12, 12 welded to the respective ends thereof, and a rolled-up electrode unit 2 encased in the can 1. A pair of positive and negative electrode terminal assemblies 9, 9 are attached to the lids 12, 12, respectively. The rolled-up electrode unit 2 is connected to the terminal assemblies 9, 9 by a plurality of electrode tabs 3, whereby the electric power generated by the electrode unit 2 can be delivered to an external device from the pair of terminal assemblies 9, 9. Each lid 12 is provided with a pressure-relief gas vent plug 13.
With reference to FIG. 13, the rolled-up electrode unit 2 comprises a positive electrode 21 containing a lithium containing composite oxide, a negative electrode 23 containing a carbon material, and a separator 22 impregnated with a nonaqueous electrolyte and interposed between the electrodes, the assembly of the these components 21 to 23 being rolled up into a cylinder. A plurality of electrode tabs 3 outwardly extend from each of the positive electrode 21 and the negative electrode 23 of the unit 2, and the outer ends 31 of the electrode tabs 3 of the same polarity are joined to one electrode terminal assembly 9. For convenience"" sake, only some of these tabs are shown as being joined to the terminal assembly 9 in FIG. 13, while the connection of the ends of the other tabs to the assembly 9 is omitted from the illustration.
The electrode terminal assembly 9 comprises a screw member 91 extending a hole in the lid 12 of the can 1 and mounted on the lid 12. The screw member 91 has a flange 92 at its base end. An insulating packing 93 is fitted in the hole of the lid 12 for electrically insulating the screw member 91 from the lid 12 and providing a seal therebetween. The screw member 91 has a washer 94 fitted therearound from outside the cylinder 11, and a first nut 95 and a second nut 96 screwed thereon similarly. The first nut 95 is tightened up to clamp the insulating packing 93 between the flange 92 of the screw member 91 and the washer 94 and thereby seal off the hole more effectively. The outer ends 31 of the electrode tabs 3 are fixedly joined to the flange 92 of the screw member 91 by spot welding or ultrasonic welding.
FIG. 9 shows a known secondary cell which is provided with a pressure relief gas vent closure 4 fitted in a through bore 14 formed in a lid 12 and operable upon the internal pressure of a cell can 1 exceeding a predetermined value (JP-A No. 68861/1994). As shown in FIG. 10(a), the gas vent closure 4 comprises a ring 41 and a disklike diaphragm 42 fixed thereto. The gas vent closure 4 is fixed to the lid 12 by joining the outer periphery of the ring 41 to an opening edge of the bored portion 14 of the lid 12 by laser welding at 15 as seen in FIG. 10(b).
With large-sized secondary cells, however, the lid 12 is as thick as at least several millimeters and has a very large thickness relative to the thickness of the gas vent closure 4. Consequently, when the closure 4 is fixed to the lid 12 by laser welding, there arises the problem that the weld develops defects, such as pinholes or cracks, because the heat of welding dissipates markedly, rapidly cooling the metal melted by being irradiated with the laser beam.
Accordingly, an object of the present invention is to provide a secondary cell wherein a gas vent closure can be welded to a lid without the occurrence of defects such as pinholes or cracks.
The present invention provides a secondary cell comprising a gastight can 1 having a cylinder 11 and a lid 12 fixed to an opening portion of the cylinder, and a secondary cell element accommodated in the can 1. The lid 12 is fixedly provided with a vent closure 4 in the form of a disk and having a diaphragm 42 openable upon the internal pressure of the can 1 exceeding a predetermined value, the vent closure 4 having an outer peripheral portion welded to an opening edge defining a through bore 14 formed in the lid 12. The lid 12 has a diminished portion 51 provided around the welded portion of the vent closure 4 for reducing the sectional area of a heat flow path for the heat of welding to pass therethrough.
Since the secondary cell of the invention described has the diminished portion 51 around the welded portion of the vent closure 4, the heat of welding is transferred first to the diminished portion 51 and then to other portion around the diminished portion 51 for dissipation. The heat capacity of the diminished portion 51 is much smaller than that of the entire lid 12, while the sectional area of the heat flow path for the heat flux to pass through is also small in the diminished portion 51, with the result that the diminished portion 51 provides resistance to heat conduction to suppress the dissipation of the heat of welding. Consequently, the weld cools at a suitable rate, solidifying without developing pinholes or cracks.
Stated specifically, the lid 12 is formed in the front surface or rear surface thereof with a circumferential groove 5 surrounding the welded portion of the vent closure 4, and the diminished portion 51 is formed between the welded portion of the vent closure 4 and a side wall of the lid defining the groove 5 and positioned closer to the vent closure 4.
Further stated specifically, the diminished portion 51 is alternatively provided by a cylindrical projection 6 jutting from the front surface of the lid 12 and surrounding the through bore 14, and the vent closure 4 is fixed to a central opening portion of the projection by welding.
Preferably, the circumferential groove 5 has a width in the range of {fraction (1/50)} of the inside diameter of the through bore 14 formed in the lid 12 to a value equal to the inside diameter, and a depth up to ⅔ of the thickness of the lid 12. If the width of the groove 5 is less than {fraction (1/50)} of the inside diameter of the bore 14, the diminished portion 51 produces a low effect, whereas if the groove width is greater than the inside diameter, there arises a problem with respect to the strength of the lid 12. Further when the groove depth is greater than ⅔ of the thickness of the lid 12, a problem as to the strength of the lid 12 will also result.
The secondary cell is, for example, at least 5 Ah to not greater than 200 Ah in capacity and 1 to 5 mm in the thickness of the lid 12. With secondary cells having such a large size or capacity, heat is markedly dissipated from the welded portion of the conventional gas vent closure, whereas the diminished portion 51 of the present invention produces an outstanding effect.
With the secondary cell of the present invention, the provision of the diminished portion 51 for the lid 12 suppresses the dissipation of heat that occurs when the vent closure 4 is welded to the lid 12, permitting the metal melted by welding to cool at an appropriate rate and therefore obviating the likelihood that the welded portion will develop pinholes, cracks or like defects.